Seepage erosion and escarpment retreat in flysch formation under mid-latitude cold climate

The development of erosional features in rockwalls indicate the transition from back-wearing (parallel retreat) erosion to down-wearing by linear dissection. Following Rapp (1960), erosional features such as chutes and funnels are the results of successive undermining rockfalls released by thawing after frost damage. The process doesn’t start from the top of the slope but over a scar leave by a large magnitude rockfall which can appear anywhere on the rock face. Sauchyn et al. (1998) suggest that the type of erosional features (chutes, funnels, open cirques) is under structural control in stratified sedimentary rock. Other, like Dunne (1990), Laity & Malin (1985), Lipar & Ferk (2015) or Duszyński et al. (2016) attributes the development of erosional features and the subsequent development of amphitheatre valley heads to underground erosion by subsurface flow. The presence of perennial spring in erosional features on the flysch rockwalls of the northern Gaspé Peninsula (Eastern Canada) suggest that these notches may be the result of seepage erosion. However, observations show that groundwater seeps out the scar of large magnitude rockfalls. If the location of the spring is conditioned by geological and structural setting, the development of erosional features may as well be under geological and structural control. To support these premises and the development of a conceptual model of erosional features formation, a detailed geological, hydrogeological and climatological characterization was carried out. Meteorological instruments, including temperature probes inserted inside horizontal borehole, were installed directly on a rockwall. Four observation wells were drilled on the plateau above the rockwall and around an erosional features. The hydraulic conductivity was determined using pneumatic slug tests performed every 1.6 meters in the boreholes. The hydraulic gradient was calculated from continuous measurements of the water table using water level logger. An optical borehole televiewer, a three-art caliper probe, an induction conductivity probe and a gamma ray detector were also used to support the geological and structural analysis of the rock mass. A structural analysis was also performed on the surface of the rockwall. The results show that the elevation and the location of the spring on the rockwall is not conditioned by a permeability contrast between geological units, the presence of a perched water table or by a network of interconnected fractures channels water. Our result show that groundwater simply reaches the rockwall surface in the erosional feature. At the periphery of the notch, the hydraulic head decreases significantly in the highly weathered and fractured rock layer near the rockwall surface. Depending on the winter severity, seasonal freeze thaw cycles reaches three to five meters deep into the rockwall. Water seepage and ice formation in the erosion notch maintain the rock temperature above 0^oC during winter. Large magnitude rockfalls mainly occur in spring after thawing and groundwater supply near the rockwall surface. The initiation and the development of erosional features in flysch formations appear to be controlled through the interaction between groundwater inflow near the surface of the rockwalls and the meteorological conditions that drive frost weathering.